Multiple types of tires comprise treads having narrow grooves in their tread blocks or tread ribs. In particular, in modern long haul drive tires such narrow grooves or sipes are part of the tread design. In general, such sipes may increase mileage and fuel economy so that they have often been applied in the past.
However, it has been observed that under heavy duty or extensive use such sipes may provoke fatigue cracks starting to propagate at the bottom of the sipes.
To overcome this problem, typical solutions of the prior art utilize an increased groove bottom radius of the sipes, e.g. by using ball-bottom blades in the mold. However, this is only a partial solution which leads to losses of design parameters and may result in drawbacks in relation to driving mileage and fuel economy. Moreover, the production of respective blades can be difficult and expensive.
European patent application EP 1 167 084 A2 discloses a tire comprising a tread portion having a plurality of tread blocks each provided with at least one sub-groove having a special shape to improve wear resistance. In one embodiment of this tire, the bottom of the sub-grooves is inclined from the outer surface of the block toward a direction of the braking force as a main stress applied from the road surface to the respective tread block and along a forward rotating direction of the tire.
This tire may constitute an improvement with respect to earlier tread designs including sipes having essentially a perpendicular orientation with respect to the surface of the tread blocks. Nevertheless, tire performance may be reduced by the mentioned design. In particular, a considerable overhang of the tread block portions divided by the sipes exists, leading to drawbacks in performance and durability of the tire.